Directed vapor deposition of electron beam evaporant

ABSTRACT

A process for vapor depositing an evaporant onto a substrate is provided which involves: 
     presenting the substrate to a deposition chamber, wherein the deposition chamber has an operating pressure of from 0.001 Torr to atmospheric pressure and has coupled thereto a carrier gas stream generator and an electron beam gun capable of providing an electron beam at the operating pressure and contains an evaporant source; 
     impinging the evaporant source with the electron beam to generate the evaporant; 
     entraining the evaporant in the carrier gas stream; and 
     coating the substrate with the carrier gas stream which contains the entrained evaporant, and an apparatus for performing the process.

This invention was made with government support under the NASA/ARPAGrant No. HQN-11, 156-ICU awarded by the National Aeronautics and SpaceAdministration. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the directed vapordeposition of an electron beam evaporant and an apparatus for performingthe process.

2. Discussion of the Background

The recent emergence of vapor phase processing technology has led to anatom by atom capability for producing coatings and engineered lamellarmaterials (already widely practiced in molecular beam epitaxy (MBE) forsemiconductor heterostructures and for the production of Al/Al₂ O₃laminates). Today, the most widely used vapor phase processingtechnologies are physical vapor deposition (e.g., sputtering,traditional electron-beam evaporation, ion deposition, effusion cells)and chemical vapor deposition (e.g., metal organic, plasma assisted). Asnoted in the table below, both of these approaches, while useful, havesignificant drawbacks which prevent or inhibit their use for processingmany industrially important materials into desired thin and thick films.

    ______________________________________                                        Advantages       Problems                                                     ______________________________________                                        Chemical Vapor Deposition (CVD)                                               High throwing power (i.e., not                                                                 Expensive precursor gases                                    line of site)                                                                 Many metals, semiconductors                                                                    High temperature needs                                       and ceramics                                                                  Uniform deposits Low deposition rates                                         Controllable microstructure                                                                    Inefficient materials utilization                                             Not all material systems                                                      Complex equipment for multi-                                                  layers                                                                        Environmental impact                                                          Residual (deposition) stress                                 Physical Vapor Deposition (PVD)                                               All metals and ceramics                                                                        Expensive equipment                                          Controllable microstructure                                                                    Low deposition rates                                                          (˜10 μm/h)                                                           Inefficient materials utilization                                             (1-25%)                                                                       High vacuum (10.sup.-7 Torr) needed                                           Line of site deposition                                                       Residual (deposition) stress                                                  Porosity                                                                      Batch process                                                ______________________________________                                    

Conventional CVD and PVD have numerous limitations. CVD often requireshigh temperatures to facilitate gas reactions while being able todeposit materials only slowly and inefficiently. Similarly, PVDprocessing requires expensive equipment and high vacuum facilities whileonly being capable of relatively low deposition rates, inefficientmaterials utilization, and line of site deposition. Traditional electronbeam evaporation technology (a type of PVD) has been utilized formaterials processing only in high vacuums, precluding its use with astream of gas to focus and direct the evaporant for efficient materialsdeposition at extremely high rates. (Within the context of the presentapplication the term "high vacuum" is equivalent to extremely lowpressure, such as 10⁻⁶ Torr, while "low vacuum" is equivalent to higherpressures from 0.001 Torr up to but not including atmospheric pressure).

JVD (jet vapor deposition, U.S. Pat. No. 4,788,082), a process whichutilizes a stream of gas to direct evaporant material to a substrate,has also fallen victim to similar limitations as CVD and JVD. The JVDprocess has always created its vapor inside of a jet forming conduit andthen directed the gas and evaporant out through a nozzle and onto asubstrate. Evaporating inside the gas flow tube is likely to lead toclogging of the nozzle even after short operation times. The inventor ofJVD only envisioned making use of fairly rudimentary evaporationtechniques such as resistive heating which are incapable of evaporatingimportant refractory elements and compounds, can operate only at lowrates (relative to e-beam evaporation), and are likely to contaminatethe deposited material as the tungsten filament or its protectivesheathing evaporates with the evaporant source material.

The inventor of JVD envisioned using only fairly small nozzles of anexit diameter "from several mm to 2 cm" ("Handbook of DepositionTechnologies for Films and Coatings", 2nd ed., Ed. R. F. Bunshah, NoyesPublications, p. 823, 1994). These relatively small nozzles limit thevolume of evaporant which may be passed through the nozzle per unit timewithout the formation of clusters via 3 body collisions. Clusters aregroups of atoms which have joined together while in the carrier gasflow. In the Handbook of Deposition Technologies for Films and Coatingsthe inventor of JVD shows the dependence of clustering upon metal vaporconcentration:

    τ.sub.38 ≡10.sup.32 /(M)(He) (sec)

where M and He are the relative concentrations of metal vapor andcarrier gas. As the concentration of metal vapor increases the timenecessary for cluster formation decreases. While this simple formulaprovides a general relation between cluster formation rate and metalvapor concentration it neglects the temperature dependence of thephenomenon as noted by Mikami. ("Transport Phenomena in Free-JetExpansions", H. Mikami, in Bulletin of the Research Laboratory forNuclear Reactors, vol. 7, p. 169, 1982). Research (D. Hill, MastersThesis, University of Virginia, p. 59, 1994) has shown this criticaltemperature dependence makes cluster formation extremely likely usingcurrent JVD technology at or near room temperature. Research resultspresented by Hill indicate that under low Mach flow regimes, as many as15-20% of all metal atoms can be involved in clustering duringdeposition at or near room temperature. When these atom clusters aredeposited on the substrate they inhibit atomic motion vital to theformation of fully-dense and crystalline deposits which are almostalways the preferred end product. To avoid clustering in theseconventional JVD systems, evaporation rates must be lowered.

For over thirty years electron beam guns (e-guns) have been recognizedand used as superior evaporation tools for producing material vaporstreams of new and unusual substances (especially of refractory metals)for deposition upon various substrates. Advantages of electron beam(e-beam) evaporation include high evaporation rates, freedom fromcontamination, precise beam (power and position) control, excellenteconomy, and high thermal efficiency. The high evaporation rate andthermal efficiency of e-beam systems are related to the ability ofe-guns to bring the heating source (electrons) directly into contactwith the vapor emitting surface where beam controls allow preciseevaporation rate control. With an e-beam source, the directly heatedvapor-emitting surface has the highest temperature of the evaporatingassembly, allowing the evaporation of materials from water-cooledcrucibles, a near necessity for evaporating reactive, and highlyreactive refractory materials. Importantly, the use of the e-beam sourcewith a water-cooled crucible allows "skull" melting/evaporation whichprevents crucible wall materials and related reaction products fromentering the vapor stream.

Crucibleless (levitation) methods are also available forcontaminant-free evaporation. Currently, such levitation methods areused to melt (alloy) small quantities of reactive metal alloys inlaboratory environments. They are also used in the aluminum industry tocontinuously cast metals. In all other heating modes the energy flowgoes through the crucible (or resistance-heated wire), then the moltenevaporant, and finally to the vapor emitting surface, allowing forsignificant thermal losses and contamination.

E-beam sources can be used to evaporate many different forms ofmaterial, whilst feeding, filling, and changing from one evaporant toanother can be easy and continuous. Pure elements, compounds, alloys,and mutually insoluble materials--virtually the entire periodic table ofelements in all possible combinations--can be processed by e-beamevaporation. Low vapor pressure elements, such as molybdenum, tungsten,and carbon, are readily evaporated, as are the most reactiveelements--titanium, niobium, and tantalum. Even alloys containingmaterials with significantly varying vapor pressures can sometimes beevaporated successfully. For example, it has been shown by others thatseemingly difficult to process Ti alloys (such as those containingvanadium) having elemental vapor pressure ratios as high as 1000:1 canbe deposited with the starting alloy's elemental ratios. However,elemental segregation continues to be a problem when working withcertain material systems, e.g. Nb-Ti.

Traditionally, e-beam evaporation has been conducted in high vacuumsystems (<10⁻⁶ Torr), allowing free propagation of both the electronbeam and the vapor stream. The resulting "unfocused" evaporation hasalways resulted in the waste of significant amounts of "expensive toproduce" source material. While the desired substrate may be an array of150 μm diameter fibers, the vapor from a traditional e-beam sourceleaves the feed stock with a density distribution often described by acos^(n) θ function (where n=1, 2, 3, or more) which results in coatingnonuniformity for large area arrays and poor materials utilization withfibers arrayed only in the region of roughly uniform flux.

Directed vapor deposition has been employed by others, but only with theuse of extremely high temperatures and with vapor phase processingmethods other than electron beam (see Kalbskopf et al, U.S. Pat. No.4,351,267; Ahmed, U.S. Pat. No. 4,468,283; and Schmitt, U.S. Pat. No.4,788,082).

Kalbskopf et al, U.S. Pat. No. 4,351,267, disclose an apparatus fordepositing a layer of a solid material on a heated substrate. Theirprocess uses a directed vapor deposition process using multiple vaporcurtains which converge on the surface of the substrate. The coatingsubstrate is provided by reactions of the gaseous reactants containedindividually in each of the vapor curtains, which when converged, reactto form the coating material.

Ahmed, U.S. Pat. No. 4,468,283 discloses a method for directed CVD whichrequires collecting and recycling of the vapor stream in order toimprove efficiency. Further, as was done by Kalbskopf et al above, Ahmeduses reactant gases which when in contact with one another, react toprovide the coating material.

Schmitt, U.S. Pat. No. 4,788,082, discloses a method for vapordepositing using jet stream entrainment in which the depositing materialvapor is generated by resistive heating, contact with a heated surface,or by use of a laser. The resulting vapor is contained within the gasjet nozzle, entrained in the carrier gas stream, and then passed throughthe jet nozzle into the deposition chamber. However, as notedpreviously, such generation of the depositing material vapor inside thegas jet may cause severe problems with nozzle clogging. Additionally,the Schmitt process can only work with a limited range of elements, atlow rates, and most often with fine wire sources.

While CVD and PVD are capable of producing many industrially importantmaterials, sizeable obstacles must be overcome if they are to yieldmaterials that are cost effective solutions to the design engineer'smaterial selection problems. In the case of CVD, very significant costsare associated with the precursor gases and their inefficientdeposition. In the case of PVD, inefficient materials utilization andthe need for high vacuum are significant cost factors.

To expand the viable applicability of such vapor phase processingtechnologies, processes are needed which are:

1. Capable of high deposition rates (>100 μm/min over 100's of squarecentimeters, i.e. grams/min.)

2. Controllable (well defined layer compositions, distinct interfaces,and microstructure)

3. Efficient (near 100%) in materials utilization

4. Flexible (allowing film layer thickness modulation, volume fractionvariation, and many materials systems to be codeposited, i.e., to allowalloying and functional grading)

5. Able to produce near net shapes (to achieve economies by combiningprocess steps)

6. Not capital equipment intensive

7. Not operator intensive (continuous, automated and reliable)

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to provide a processfor directed vapor deposition using an electron beam evaporant whichmakes efficient use of evaporant source materials and provides rapiddeposition.

A further object of the present invention is to provide a process fordirected vapor deposition which uses an electron beam evaporant and hashigh flexibility in its use by allowing film layer thickness modulation,volume fraction variation, and functional grading in a co-depositionsystem.

Another object of the present invention is to provide a process fordirected vapor deposition of an electron beam evaporant which iscontinuous, automated, reliable and neither operator nor capitalequipment intensive.

A further object of the present invention is to provide a process fordirected vapor deposition of an electron beam evaporant which operatesat low vacuum conditions.

A further object of the present invention is to provide a process fordirected vapor deposition of an electron beam evaporant which is capableof producing near net shapes, thus achieving economies by combiningprocess steps.

A further object of the present invention is to provide a process fordirected vapor deposition of an electron beam evaporant which providessufficient control to give well defined layer compositions, distinctinterfaces, residual stress control, and microstructure control.

A further object of the present invention is to provide a processcapable of continuous, directed deposition over a large area at a highrate, without clustering.

A further object of the present invention is to provide a processcapable of utilizing a bias voltage to obtain ion assisted directedvapor deposition (IADVD).

A further object of the present invention is to provide an apparatus toperform directed vapor deposition of an electron beam evaporant whichuses an electron beam gun capable of providing an e-beam at low vacuumconditions.

These and other objects of the present invention have been satisfied bythe discovery of a process for directed vapor deposition of an electronbeam evaporant, onto a substrate comprising:

presenting the substrate to a deposition chamber, wherein saiddeposition chamber has an operating pressure of from 0.001 Torr toatmospheric pressure and has coupled thereto a means for providing acarrier gas stream and a means for providing an electron beam at saidoperating pressure and contains an evaporant source;

impinging said evaporant source with said electron beam to generate saidevaporant;

entraining said evaporant in said carrier gas stream; and

coating said substrate with said carrier gas stream which contains saidentrained evaporant.

DESCRIPTION OF THE FIGURES

A more complete appreciation of the present invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 is a schematic representation of an apparatus used to perform thepresent process.

FIG. 2 shows a preferred embodiment of the apparatus used in the presentprocess.

FIG. 3 shows a cutaway view inside the deposition chamber of theapparatus of the present invention.

FIG. 4 shows an apparatus for fiber manipulation in the depositionchamber of the present process.

FIG. 5A is a schematic representation of an apparatus for performing theprocess of the present invention continuously.

FIG. 5B is a schematic representation of a pressure decoupling chamberfor use in the practice of the present continuous process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a process for vapor deposition of anevaporant onto a substrate comprising:

presenting the substrate to a deposition chamber, wherein saiddeposition chamber has an operating pressure of from 0.001 Torr toatmospheric pressure and has coupled thereto a means for providing acarrier gas stream and a means for providing an electron beam at saidoperating pressure and contains an evaporant source;

impinging said evaporant source with said electron beam to generate saidevaporant;

entraining said evaporant in said carrier gas stream; and

coating said substrate with said carrier gas stream which contains saidentrained evaporant.

In particular, the present invention makes possible the creation of thinand thick films of any element or combination of elements at lowervacuum than previously possible by traditional electron beamevaporation. It also makes possible the creation of films from elementsor combinations of elements which current low vacuum vapor depositiontechniques are incapable of processing because of their inability tosufficiently heat the material source for rapid evaporation.

By combining e-beam evaporation in a partial inert gas pressure (lowvacuum=1-10 Torr) with a purified direct gas flow, it is possible to usethe valuable source materials much more efficiently and to depositmaterials much more rapidly than is currently feasible via eithertraditional e-beam evaporation or other low vacuum vapor depositionsystems. Instead of line-of-sight deposition, uniform, directeddeposition on any desired substrate in any position is possible.

By operating an electron beam system at low vacuum in accordance withthe present invention, a processing capability far superior to any othercurrently in existence has been achieved. The process of the presentinvention can evaporate and deposit reactive materials, (e.g., oxides,nitrides, and carbides) at high rates, simultaneously deposit differentelements in precise elemental ratios for alloying purposes, andalternate between alloys for the production of microlaminates.

One problem with current e-beam evaporation facilities which attempt toevaporate alloys is that different parts of the evaporant pool havedifferent composition due to temperature gradients in the evaporantpool. This results in nonuniform alloying ratios in the deposit. Byentraining the evaporant in a turbulent directed gas flow, it ispossible in the present process to mix the segregated vapor streams andcreate a uniform alloy deposit.

The evaporant in the present invention is generated by impinging theevaporant source with an e-beam such that the vaporized evaporant isgiven off and entrained in the carrier gas stream. The step of impingingthe evaporant source with the e-beam can be performed, if desired,inside of the means for generating the carrier gas stream. Thus, thee-beam would enter into the carrier gas conducting assembly which wouldalso have the evaporant source contained therein. Once the evaporant isformed it would then be entrained in the carrier gas stream prior to anynozzle, pass through such a nozzle and be directed to the substrate.

However, when creating the evaporant prior to a nozzle, it is necessaryto avoid clogging the nozzle. The conventional JVD process almostcertainly encounters problems with nozzle clogging when the evaporant isprepared inside the jet prior to the nozzle. While methods forpreventing such nozzle clogging have been suggested, such as using up to50% of hydrogen in the carrier gas stream, slower evaporation of theevaporant source and increased gas flow, such solutions also haveundesirable effects on efficiency. The use of hydrogen in the carriergas stream is unsuitable when using refractory materials such astitanium due to the formation of hydrides. Slower generation of theevaporant has obviously detrimental effects on the deposition rate.Increased gas flow wastes carrier gas sources and can cause an increasein the absolute level of impurities deposited on the substrate.

In order to generate the evaporant of the present invention inside ofthe carrier gas conduit, prior to the nozzle, it is preferred to use agas conduit and nozzle assembly with a diameter of 2.5 cm or greater, asopposed to the conventional nozzles used having diameters less than 2cm. This increase in conduit and nozzle diameter helps to preventclogging and provides a wide carrier gas stream which can increaseefficiency and deposition rate by covering more area per unit time. Notethat in certain instances it may be desirable to use a gas conductingtube with no nozzle, i.e., a straight pipe.

While the carrier gas conduit can consist of a straight flow tube, itmay be desirable to at least partially enclose the end of the flow tubewith a type of nozzle, such as a converging, diverging orconverging/diverging nozzle. In addition, such nozzles can be designedto allow for real-time change of the nozzle shape and/or dimensions.This flexibility increases the utility of the processing system of thepresent invention especially during multilaminate or continuousprocessing where optimal coating of the substrate with a given materialcan require differing flow conditions during the processing.

In a preferred embodiment of the present invention, the step ofimpinging the evaporant source with the e-beam is performed inside thedeposition chamber, but external to the means for generating the carriergas stream. By performing the process in this manner, the present systemeliminates the need for modifying the carrier gas by inclusion ofhydrogen. Further, this embodiment avoids the requirement of using alarge diameter jet nozzle, although use of such a large diameter nozzlecan still provide deposition rate improvements with this embodiment.

Evaporation via resistive heating may work well for low melting pointmaterials such as aluminum and gold where the heating element (oftentungsten) can supply the necessary heat energy for fairly rapidevaporation rates. However, resistive heating is wholly unsuitable foruse with reactive, refractory materials such as titanium or molybdenum.In this case, evaporation rates are slow and contamination of thedesired evaporant by the heating element proves to be a problem. Inaddition, failure of resistive heating elements limits the utility ofsuch systems for continuous industrial production facilities. Asexplained below, the use of an electron beam gun as the heating sourcemakes possible the rapid, efficient evaporation of any and all materialsduring long, continuous runs.

Finally, the operating characteristics of the new apparatus make ithighly suitable for industrial-scale production purposes. The apparatusfor performing the present process can be brought to a suitable pressurein less than 5 minutes, allowing for rapid changing of source,substrate, or other system components during maintenance or systemreconfiguration shutdowns. Traditional e-beam systems require asubstantially greater length of time to pump down into their operatingpressure range of below 10⁻⁶ Torr.

The mean time between failures of the filament in the electron beam gunof the present invention is at least 10 hours when operating the gunconstantly at full power (10 KW). This is far longer than the expectedlifetime of the resistive heating elements used by current low vacuumevaporation facilities. In general resistive (contact) heating sourcesare not well suited to use in continuous processing systems. As is wellknown by operators of heating elements and light bulbs, contact of theheating/lighting source with any substance, especially reactive (e.g.,oxidizing) agents, leads to rapid deterioration and failure of thesource in many instances. Such failures necessitate frequent, timeconsuming maintenance using conventional resistive heating elements.Such a maintenance requirement is relieved by the present apparatus andprocess through the use of an electron generating filament in the highvacuum (clean) portion of the gun.

The unique and potentially revolutionary process of the presentinvention combines a directed gas flow process with electron beamheating to evaporate and deposit metal and ceramic vapors onto flat andfibrous substrates efficiently. The directed evaporation and depositionsystem of the present invention has proven that an electron beam gunsource can vaporize metals in a vacuum chamber at low vacuums such as 5Torr, or even up to atmospheric pressure, and the metal vapor createdwith this e-beam heating source can be successfully entrained in adirected flow of helium gas and deposited on glass slides, producing awide range of products from thin shiny deposits to thicker,rough-surfaced metal films.

The process of the present invention has enormous potential as asolution for rapidly and economically producing conventional coatedmaterials (e.g. TiN coated titanium) and new products such as highquality multilayered thermal barrier coatings, large area flat paneldisplays, high temperature superconductors, thin film photovoltaics, andmultichip modules. Processing of advanced materials via this techniquerequires careful real-time monitoring and control of the evolvingmicrostructure critical to the performance of the final product. Thiscan be achieved by optimization of the process parameters to provide thedesired material microstructure and product properties needed for thespecific application.

The directed vapor deposition system of the present inventionincorporates a 60 KV/10 KW electron beam gun source into its fundamentaldesign. The present electron beam gun electronic controller and powertransformer were built based upon theoretical calculations concerningthe generation of an e-beam in low vacuum and how far such an e-beamwould travel under such conditions. From these calculations, the presentapparatus was designed.

The major components of the directed vapor deposition system describedherein are shown in the schematic assembly drawing of FIG. 1. Theelectron beam gun assembly is composed of three major components: thegun itself (30), the gun's power supply (40), and the gun's electroniccontrol system (10). The power supply (40) is an electrical transformerwhich steps the voltage from the voltage available to the 60,000 Vneeded by the gun. The electronic control system (10) ensures proper gunoperation by controlling the gun's beam focussing coils, monitoring thecurrent flow through the cathode, anode, and nozzle, ensuring that thecross-valve between the high vacuum and differentially-pumped portionsof the gun is open, and allowing the transformer to supply power whenall safety checks have been satisfied. The electronic control system canbe operated either manually or preferably using a computer.

Since this directed vapor deposition system is a vacuum process, itrequires the use of vacuum pumping assemblies, such as a high vacuumturbomolecular pump (20) with a mechanical backing pump. This pumpingpackage combination is capable of maintaining a vacuum greater than9×10⁻⁷ Torr in the electron beam generating portion of the gun when thesystem is not in operation and greater than 8×10⁻⁵ Torr when it is inoperation. The differential pumping package (50) ensures that the vastmajority of the carrier gas from the deposition chamber (100) does notreach the beam generating space during processing by instead removingthat gas to the outside atmosphere. The third pumping package (60) isattached to the deposition chamber (100) and is responsible for removingthe carrier gas introduced to the system through the carrier gas streamgenerating means (70). Once the three pumping packages are operational,the electron beam gun (30) can be turned on and a beam generated. Thisbeam emerges from the gun column into the deposition chamber (100) andimpinges upon the evaporant material source (80). Once heated, thissource (80) emits a vapor stream which is captured by the carrier gasstream and carried towards the substrate (90) where most or all of theevaporant is deposited. All processing occurs inside the depositionchamber (100), preferably made of thick-walled stainless steel.

FIG. 2 provides a more detailed illustration of a preferred arrangementof the components of the directed vapor deposition system. In thisembodiment the system is shown in operation with the beam (31) impingingupon the evaporant material source (80). Then the created vapor istransported by the carrier gas stream (71) to the substrate (90), inthis instance an assembly of fibers.

The deposition chamber (100) is equipped with multiple sensing ports(101) necessary for closed-loop process control and optimization.Important in-situ sensing information obtained through sensing ports(101) includes temperature, pressure, deposit uniformity, growth rate,stress, microstructure, carrier gas flow characteristics (e.g.,turbulent versus laminar, fast versus slow), and molten pool heightdata. As an example of the utility of such information it is noted thatsensing of the size and shape of the molten pool can be used todetermine the rate at which the source material should be fed into thecarrier gas stream and/or chamber. Sensors used for all thesemeasurements can include thermocouples, resistance temperature detectors(RTD's), FLIR cameras, assorted pressure gauges (e.g., cathode,thermocouple, diaphragm), optical sensors, laser induced fluorescencedevices, ultrasonic sensors, eddy current sensors, microwave sensors,and x-ray diffractometers (XRD).

The deposition chamber (100) has an electron beam evaporant source (80)and the capability to incorporate other PVD and CVD sources oradditional e-beam sources. The use of a box type deposition chambermakes possible the inclusion of door (102) which is the full size of oneside of the deposition chamber (100). Door (102) provides quick and easyaccess to the entire interior of deposition chamber (100) for rapidchanging of evaporant source (80), substrate (90), carrier gas streamgenerator (70), and other interior components such as heating lamps (notshown). The outside of the entire chamber (100) can be equipped with awater cooling conduit (not shown) which keeps the system's walls safelycooled while processing inside the system can occur at elevatedsubstrate and gas temperatures. Additionally, because the entrainedevaporant does not spread throughout the chamber as it does intraditional electron beam evaporation it is possible to equip depositionchamber (100) with large (5-15 cm diameter or larger) continuous viewingports (103). Viewing ports (103) do not need to be shuttered to preventcoating and thus are available for continuous optical or laser sensing.These larger sensing/viewing ports (103) can be composed of areplaceable glass layer (to absorb what small amount of coating doesoccur) inside of a sealed quartz and/or lead-glass viewing port forvacuum integrity and x-ray protection.

The rate at which gas flows through the system can be regulated bothupstream (before the carrier gas stream generator (70)) and downstream(outside the deposition chamber (100) but just before the chamberpumping system (60)). Upstream pressure control can be performed via amass flow sensor, valve and controller while downstream pressure controlcan be performed via a throttle plate located in front of the chamberpumping package. The mass flow components regulate how much gas entersthe system per unit time (i.e., standard liters per minute) while thethrottle plate determines how rapidly the gas can be pumped out of thechamber. The mass flow components, the throttle plate, and the settlingchamber make up the pressure level and pressure ratio control apparatus,central to determining whether flow is super- or subsonic. Additionally,the interior and exterior of the walls of deposition chamber (100) havebeen provided with threaded holes (not shown) which traverse onlypartially through the chamber wall, allowing process monitoringequipment, substrates, substrate heaters, source monitors, etc., to bemounted either inside or outside the chamber. Finally, a computer (110)is coupled to the apparatus which is used for data acquisition andcontrol.

While the preferred embodiment depicted in FIG. 2, shows the e-gunmounted in-line with the evaporant source, this need not be the case. Asis well known to those skilled in the e-gun art, the e-beam generatedcan be easily redirected after emerging from the gun. This allows theadded flexibility of mounting the gun in any orientation so long as thee-beam impinges upon the evaporant source material generally from above.This ensures that the molten pool formed by e-beam heating does not dripor slide off the top of the source material.

In the present system, the electron beam propagates through a tubecontaining a partial pressure of helium (from 10⁻⁶ Torr in the beamgenerating space to 10⁻² Torr in the differentially pumped lower portionof the gun) and can evaporate the source material from either a cooledwire, water cooled copper crucible feed system, or contactless, coldcrucible. By attaching this heating source to a vacuum chamber(containing a partial pressure of 1 millitorr to 5 Torr of helium), ametal vapor can be created beyond the exit of the helium gas flow tube,entrained within that flow, and directed onto a substrate.

A number of variables can be manipulated in performing the process ofthe present invention. These variables include vapor entrainment rate,evaporation and deposition rates, gas flow velocity from the carrier gasjet, chamber pressure, bias voltage (polarity) rate of bias voltagemodulation, and substrate temperature.

The evaporation rate, ER, and deposition rate, DR, combine to give theefficiency, E_(ff), of the process in accordance with the followingformula: ##EQU1##

The following Table 1 provides the evaporation rates possible for avariety of elements using an e-gun operating at 10 KW power.

                                      TABLE 1                                     __________________________________________________________________________    Elements                                                                      Al       B   C   Mo  Ni Nb  Ti  V   Zr  Y                                     __________________________________________________________________________    mole/sec                                                                           2.69                                                                              0.016                                                                             0.010                                                                             0.014                                                                             0.02                                                                             0.015                                                                             0.018                                                                             0.018                                                                             0.015                                                                             0.020                                 cm.sup.3 /min                                                                      1605                                                                              4.35                                                                              3.3 7.65                                                                              9.9                                                                              7.95                                                                              11.55                                                                             11.7                                                                              12.6                                                                              23.6                                  g/min                                                                              4333.5                                                                            10.18                                                                             7.45                                                                              78.18                                                                             88.1                                                                             68.1                                                                              52.4                                                                              71.5                                                                              82.0                                                                              105.3                                 __________________________________________________________________________

The gas flow velocity of the carrier gas jet is manipulated by the useof a pressure differential from the upstream pressure P_(o) in the jet,to the background or deposition chamber pressure P_(b), as described in"Atomic and Molecular Beam Methods," Vol. 1 Ed. Giacinto Scoles, OxfordUniversity Press, pp. 14-15 (1988). In particular, the gas flow velocitywill reach sonic levels of Mach 1 or greater when P_(o) /P_(b) >2.1 forall gases. In the process of the present invention it is preferable tomaintain a gas flow velocity wherein P_(o) /P_(b) is from 1 to 1000,preferably 1.2 to 10, most preferably at the sonic/subsonic border ofP_(o) /P_(b) ≈2.1.

In the process of the present invention the deposition chamber ismaintained at a pressure from 0.001 Torr to atmospheric pressure.Preferably the deposition chamber is maintained at a low vacuum of from0.01 to 100 Torr, most preferably 0.1 to 10 Torr.

The substrate used in the present invention can be maintained at anytemperature suitable for effecting deposition, such as from -196° C. to1500° C., preferably 0° to 900° C., most preferably 400° to 700° C. Ofcourse the temperature chosen depends on the nature of the substratesince the substrate must be physically and thermally stable duringdeposition. Thus it is necessary to avoid temperatures which causepermanent embrittlement for a given substrate and avoid temperatures atwhich the substrate thermally decomposes. Despite these requirements, itis conceivable to deposit onto materials that have high vapor pressures,e.g. liquids.

The system employs a differentially pumped gun column to generate anelectron beam in a 10⁻⁶ Torr pressure zone evacuated by a high vacuumsystem such as a Balzer TPH330 (22,200 l/min @10⁻⁵ Torr) double flowstandard turbomolecular vacuum system. Once created, the electron beamis transmitted with minimal energy loss down the gun column into a 10⁻²Torr pressure region evacuated by a second vacuum system operating atmuch lower vacuum such as an Edwards Model EH500 (8500 1/min @10⁻² Torr)mechanical booster pumping package. Finally, the beam emerges into thedeposition chamber through a hole in a replaceable plug, made of ametal, metal alloy or other elemental material which has a melting pointhigh enough to avoid melting caused by impingement of a portion of thecollimated e-beam on the plug. This ensures that frequent replacement ofthe plug is not necessary. Suitable materials for the plug have meltingpoints above approximately 2000° C. and include metals and alloys suchas tungsten, molybdenum, tungsten/zirconium/molybdenum (TZM), niobium,nickel alloys, nickel superalloys, stainless steels, zirconium, hafniumand hafnium carbides and elemental materials such as carbon. The plug ismost preferably made of tungsten. The replaceable plug separates the guncolumn from the process chamber and holds the deposition chamber at thedesired operating pressure using a low vacuum system such as a StokesModel 1722 (30,000 l/min @1 Torr) blower package.

As the beam propagates down the gun column it is focusedelectromagnetically into an intense e-beam having sufficient energy anddiameter to penetrate through the partial pressure of the system to theevaporant source. The directed carrier gas must be a gas which allowspenetration of the electron beam from the gun to the source material andis preferably He or a mixture of He and one or more gases selected fromO₂, N₂, hydrocarbons, silanes, and other non-He inert gases. Suitablehydrocarbon gases include methane and acetylene. It is also possible toutilize all the traditional CVD precursors and use the e-beam todecompose them.

FIG. 3 shows a cutaway view inside the processing chamber of the presentsystem. In this figure, the specific interaction between the electronbeam (31), evaporant source (80), and carrier gas (71) can be observed.The vapor so produced and entrained can then be deposited onto thesubstrate (90), in this case onto an array of fibers. It is alsopossible from this figure to observe the location of one of the largecontinuous viewing ports (103). Such a port makes possible in-situsensing (of the gas flow, electron beam (31), and evaporant source (80))vital for optimal system operation.

The evaporant source is located in line with the e-beam and issufficiently close to the e-gun to allow penetration of the e-beam fromthe gun, through the carrier gas, to impinge on the evaporant source.This distance is dependant on such factors as the e-beam energy and thedensity of the carrier gas. The evaporant source is located 0.5 cm to 8m from the bottom of the e-gun column, preferably 1 to 20 cm, mostpreferably 1 to 5 cm. When the evaporant source is located at a distanceof greater than 30 cm, then it is preferred to use magnetic fields atvarious locations along the length of the beam to maintain the beam'sfocus. Such magnetic fields may be generated by toroidal electro-magnetsor other electromagnetic means.

The intense e-beam preferably has an energy of up to 170 mA for a 10 KWsystem or correspondingly higher for higher power (wattage) e-guns. Thefocused beam diameter at the evaporant source is preferably 0.4 mm to 10cm, more preferably 0.4 mm to 2 cm, most preferably 1 cm. In order toprovide beam diameters of greater than about 1 mm, a magneticdefocussing means can be used. Suitable magnetic defocussing meansinclude a toroidal electromagnet having a field which operates todecollimate the beam just prior to impingement on the evaporant sourceallowing evaporation of the source at higher rates and avoidinglocalized overheating of the source.

As an alternative embodiment, the e-beam can be rastered to providedistributed heating of the evaporant source. Such e-beam rastering ispreferably performed by use of electromagnetic deflection of the e-beamusing conventional deflection coils and conventional rastering patterns.It is also possible to keep the e-beam stationary and raster/rotate theevaporant source material instead.

Given the sharp focus of the electron beam and the short propagationdistance to the source feed material in the preferred embodiment,efficient heating and evaporation of any desired feed stock from a wirefeed mechanism, water-cooled rod-fed crucible, or contactless, coldcrucible is possible in gettered He partial pressures of up toatmospheric pressure preferably and most preferably at pressures of upto 5 Torr.

The gas pressure in the various portions of the system can be preciselymeasured by an array of vacuum gauges, preferably computer monitoredvacuum gauges. While the pressure in the beam generating space ismeasured by a Penning gauge (also called a hot or cold cathode gauge)and a Pirani gauge (also called a thermocouple gauge), the pressure inthe differentially pumped lower portion of the gun is measured by asingle Pirani gauge. The pressure in the main processing chamber, heliuminlet tube, and gas settling chamber is measured by separateconventional capacitance manometer gauges each of which utilizes agas-deformable membrane to gauge pressure independent of gascomposition. This type of highly accurate, gas independent gauging isvaluable given the various gas mixing regimes which can be utilized toproduce products such as metal/ceramic functionally graded composites.The pressure measurements from these gauges are used as input to aconventional mass flow controller and valve system, such as an MKS Model647B mass flow controller and valve system. This computer-controlledflow monitoring system uses the pressure readouts from the vacuum gaugesas input for determining the desired gas flow rate. The gas flow systemcan precisely mix multiple gases, preferably up to eight different gassources, allowing vast flexibility when choosing reactive mixtures fordeposits.

For the most efficient evaporation from a crucible source, either anelectromagnetic deflection system can be used to deflect the electronbeam and distribute the beam's power over the surface of the (0.5-20 cmdiameter) rod stock or a toroidal electromagnetic defocussing system canbe used to spread out the beam's power once it passes through thenecessary constriction (2.5 mm diameter hole) at the bottom of the guncolumn, as previously described. Such a deflection system will ensurethat the source material leaves the rod stock as an atomistic vaporrather than as large droplets of molten material as the beam power isincreased. In the present system, metal vapor entrainment occurs outsidethe gas conducting tube, through which ultrapure carrier gas(preferably>99.9995% pure), such as gettered helium or other carriergasses, can flow, eliminating any problems associated with nozzleclogging. In addition, the substrates can be heated to temperatures of600° C. and above via a computer controlled quartz heating lamp andresistance temperature detector (RTD) temperature sensing system. Theability to control the temperature of the depositing species preciselyallows exploration of the structure and properties of the deposit. Thecombination of focused electron beam evaporation, focused gasentrainment of the evaporated species, and substrate heating representsa new materials processing path with unrivaled potential for theefficient high speed production of new or previously difficult to makematerial systems.

The process of the present invention can be used to coat any substratesuitable for use in vapor deposition processes. The process can thus beused to provide fibers for high performance metal matrix composites andto prepare multilayered materials with functionally graded properties.Various (e.g. ultrasonic, microwave, eddy current, XRD) in-situ sensormethods can also be used for characterizing deposit uniformity, growthrate, microstructure, and other important properties during each of thedeposition processes.

In an alternative embodiment, ion-assisted directed vapor deposition(IADVD) can be performed using the process of the present invention byplacing an electrostatic bias voltage on the substrate or behind thesubstrate, relative to the incoming evaporant/carrier gas stream. Sincethe evaporant contains ions and due to its interaction with the e-beam,this electrostatic bias provides further fine control of microstructureand characteristics of the coating by allowing the energy of the carriergas entrained evaporant to be modulated just prior to impinging on thesubstrate. Depending on the bias (either negative or positive) theenergy of the evaporant/carrier gas stream can be decreased orincreased. By increasing the energy of the evaporant/carrier gas stream,the ions impinging the substrate then have sufficient additional energyto allow atomic rearrangement of the coating. This atomic rearrangementassists the formation of crystalline coatings.

The bias voltage for use in the IADVD embodiment ranges from unbiased (0KeV) to a bias of 10 KeV, either positive or negative. Of course, inorder to apply a bias to the substrate, the substrate must be capable ofsustaining an electrical bias. Alternatively, a biasing means can beprovided which is located behind the substrate relative to the sidewhich is impinged by the evaporant/carrier gas stream. Suitable biasingmeans would include metallic plates or other conducting orsemiconducting objects which are capable of sustaining an electrostaticbias.

The bias can be applied either continuously or can be modulated, ifdesired.

The process of the present invention can be performed either in abatchwise, semicontinuous or continuous manner. In performing a batchprocess, the substrate is placed into the deposition chamber, theapparatus sealed and evacuated to the desired operating pressure. Thee-beam and directed carrier gas stream are then activated to begin thedeposition process.

In the semicontinuous embodiment, a substrate is needed which can bemoved through the carrier gas/evaporant stream. This embodimentpreferably relates to the coating of substrates which can be rolled ontoa spool, such as thin metal strips or fibers. FIG. 4 shows one preferredembodiment of the system in which the entrained evaporant sourcematerial is deposited onto an array of fibers (91) which are translatedand rotated in front of the vapor stream (71), allowing forsemicontinuous processing. The gearing mechanisms (120 and 125) to theleft and right, respectively, of the vapor stream are coupled togetherso that both sets of gears turn in unison. This arrangement allows thefibers (91) to be pulled back and forth in front of the vapor stream(71). This ability to move back and forth is important when depositingmultilayered coatings. It is possible to translate all of the fibers(91) on the spools (130) in front of the vapor stream (71) and coat thembefore changing evaporant sources and drawing the fibers (91) back infront of the vapor stream (71). This process can be repeated until alldesired layers have been deposited. To ensure uniform coating aroundeach fiber, the entire fiber manipulation gearing mechanism can rotateabout an axis along which the center fiber runs. This arrangementensures that the fibers remain in the vapor stream at all times whilebeing coated symmetrically.

The process of the present invention can also be performed continuouslyas shown in FIG. 5A by introducing a substrate (90) continuously througha pressure decoupled substrate entry port (140), passing the substratethrough the carrier gas stream (71) and transporting the coatedsubstrate (90) from the deposition chamber (100) through a pressuredecoupled substrate exit port (141). The evaporant source can likewisebe fed either as a continuous rod through a pressure decoupled evaporantsource port (150) or be pumped into a crucible when the evaporant sourceis a liquid (not shown). The pressure decoupled substrate entry port(140), pressure decoupled substrate exit port (141) and pressuredecoupled evaporant source port (150) use differential pumping toprovide a stable vacuum within the deposition chamber (100). FIG. 5Bprovides a schematic representation of a pressure decoupled portsuitable for use as either the pressure decoupled substrate entry port(140), pressure decoupled substrate exit port (141), or pressuredecoupled evaporant source port (150), described above. The port of thisembodiment uses a pressure decoupling chamber (142) which is connectedto the deposition chamber by way of passages (143). Passages (143) mustbe of a size sufficient for the substrate (90) or evaporant source (nowshown) to pass through. Since passages (143) are not sealed airtight,even when the substrate or evaporant source are inserted, they alsoallow for the entrainment of gas from one chamber to another. In orderto maintain a stable vacuum on the deposition chamber (100), whichoperates at pressures of from 0.001 Torr to atmospheric pressure, thepressure decoupling chamber (142) must be operated at a pressure belowthat of deposition chamber (100) and sufficient to provide flow of gasesfrom deposition chamber (100) to pressure decoupling chamber (142).Likewise, the gases from the atmosphere enter decoupling chamber (142)around the sample or evaporant source through passages (143) throughwhich the sample or evaporant source is introduced into (or extractedfrom) pressure decoupling chamber (142). The pressure in pressuredecoupling chamber (142) is preferably maintained at a value which is atleast one order of magnitude lower than the pressure of depositionchamber (100) using pump (160). In order for the present process to betruly continuous, deposition chamber (100) must have a pressuredecoupled substrate entry port (140), a pressure decoupled substrateexit port (141) and a pressure decoupled evaporant source port (150).

While the process and apparatus of the present invention is most simplyoperated with a single evaporant source, the vacuum chamber inside ofwhich processing occurs can have additional evaporant sourceintroduction ports. Further, additional e-beam sources or othervapor-deposition sources (e.g., CVD, resistively heated) can be fittedonto the chamber to expand the flexibility of the system. Suchadditional material inlets could alternate deposition with the e-beamsource for multilaminate production or could be used in concert with itto deposit alloys composed of elements with vastly different vaporpressures.

The system in operation has basic computer accessible process controlequipment attached for temperature and pressure measurement and control.Additional in-situ facilities can be incorporated into the chamber andcoupled with advanced computer control systems to ensure an optimalfinal product microstructure central to high quality productperformance.

Obviously numerous modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practical otherwise than as specifically described.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A process for vapor depositing an evaporantonto a substrate comprising:presenting the substrate to a depositionchamber, wherein said deposition chamber has an operating pressure offrom 0.001 Torr to atmospheric pressure and has coupled thereto a meansfor providing a carrier gas stream and a means for providing an electronbeam at said operating pressure and further contains an evaporantsource; impinging said evaporant source with said electron beam togenerate said evaporant; entraining said evaporant in said carrier gasstream; and coating said substrate with said carrier gas stream whichcontains said entrained evaporant.
 2. The process for vapor depositionas claimed in claim 1, wherein said impinging step occurs within saiddeposition chamber and externally to said means for providing a carriergas stream.
 3. The process for vapor deposition as claimed in claim 1,wherein said electron beam traverses said carrier gas stream prior toimpinging said evaporant source.
 4. The process for vapor deposition asclaimed in claim 1, wherein at least a portion of said evaporant sourceis located within said carrier gas stream.
 5. The process for vapordeposition as claimed in claim 1, wherein said evaporant source isentirely outside of said carrier gas stream.
 6. The process as claimedin claim 1, wherein said evaporant source is a material selected fromthe group consisting of elemental materials and mixtures thereof.
 7. Theprocess of claim 1, wherein said evaporant source is a free standingsolid material.
 8. The process of claim 1, wherein said evaporant sourceis contained in a crucible.
 9. The process of claim 8, wherein saidcrucible is a water cooled crucible.
 10. The process of claim 9, whereinsaid evaporant source is in the form of a rod and said water cooledcrucible is a continuously rod-fed, water cooled crucible.
 11. Theprocess of claim 1, wherein said evaporant source is held in place by anelectromagnetic levitation means.
 12. The process of claim 11, whereinsaid electromagnetic levitation means is a contactless crucible.
 13. Theprocess of claim 1, wherein said evaporant source is in a form selectedfrom the group consisting of a liquid, a powder, or a solid mass. 14.The process of claim 13, wherein said rod is a continuously fed rod. 15.The process of claim 13, wherein said rod is a wire.
 16. The process ofclaim 1, wherein said evaporant source is continuously fed into thedeposition chamber through an evaporant source port, wherein saidevaporant source port provides a passage from outside said depositionchamber into said deposition chamber.
 17. The process of claim 1,wherein said electron beam is applied to said evaporant source at aposition on said evaporant source which is varied by an electromagneticrastering means.
 18. The process of claim 1, wherein said electron beamis defocused after exiting said means for providing an electron beam andprior to impingement on said evaporant source.
 19. The process of claim18, wherein said electron beam is defocused by an electromagnet.
 20. Theprocess of claim 1, wherein said carrier gas stream comprises an inertgas or a mixture of an inert gas with one or more reactive gases. 21.The process of claim 20, wherein said carrier gas stream has a purityof>99.9995% for each individual gas.
 22. The process of claim 21,wherein said carrier gas stream comprises He.
 23. The process of claim22, wherein said carrier gas stream further comprises one or more gasesselected from the group consisting of O₂, N₂, hydrocarbons, silanes, andinert gases other than He.
 24. The process of claim 21, wherein saidcarrier gas stream consists of He.
 25. The process of claim 1, whereinsaid substrate has a form selected from the group consisting of flatsubstrates, multifaceted substrates, fibrous substrates, and curvedsubstrates.
 26. The process of claim 25, wherein said fibrous substrateis a member selected from the group consisting of crystalline sapphirefibers and silicon carbide fibers.
 27. The process of claim 1, whereinsaid substrate is moving during the process.
 28. The process of claim 1,wherein said substrate is continuously fed into said deposition chamberand continuously removed from said deposition chamber after deposition.29. The process of claim 1, wherein said substrate is maintained at atemperature of from -196° C. to 1500° C.
 30. The process of claim 29,wherein said substrate is maintained at a temperature of from 0° to 900°C.
 31. The process of claim 30, wherein said substrate is maintained ata temperature of from 400° to 700° C.
 32. The process of claim 1,wherein said operating pressure is from 0.001 Torr up to, but notincluding, 1 Torr.
 33. The process of claim 1, wherein said operatingpressure is from 1 Torr to atmospheric pressure.
 34. The process ofclaim 1, wherein said operating pressure is from 0.01 to 10 Torr. 35.The process of claim 34, wherein said operating pressure is from 0.1 to5 torr.
 36. The process of claim 1, wherein said electron beam isapplied to said evaporant source at a position on said evaporant sourcewhich is varied by laterally moving said evaporant source in relation tosaid electron beam.
 37. The process of claim 1, wherein an electrostaticbias is applied to said substrate.
 38. The process of claim 37, whereinsaid electrostatic bias is a positive bias.
 39. The process of claim 37,wherein said electrostatic bias is a negative bias.
 40. The method ofclaim 37, wherein said electrostatic bias is applied continuously. 41.The method of claim 37, wherein said electrostatic bias is varied withtime.
 42. An apparatus for vapor depositing an evaporant onto asubstrate, comprising:a deposition chamber; an electron beam gun coupledto said deposition chamber, wherein said electron beam gun is capable ofproviding an electron beam in said deposition chamber when saiddeposition chamber is maintained at an operating pressure of 0.001 Torrto atmospheric pressure; and a means for generating a carrier gasstream, wherein said means for generating a carrier gas stream islocated such that said electron beam passes through at least a portionof said carrier gas stream prior to impinging on an evaporant source.43. The apparatus of claim 42, further comprising a pressure decoupledsubstrate entry port and a pressure decoupled substrate entry port and apressure decoupled substrate exit port located on opposite sides of saiddeposition chamber and in alignment with one another such that asubstrate can pass linearly through said pressure decoupled substrateentry port, through said carrier as stream and through said pressuredecoupled substrate exit port.
 44. The apparatus of claim 42, furthercomprising a pressure decoupled evaporant source entry port allowing forcontinuous feeding of an evaporant source.
 45. A process for vapordepositing an evaporant onto a substrate comprising:presenting thesubstrate to a deposition chamber, wherein said deposition chamber hasan operating pressure of from 0.001 Torr to atmospheric pressure and hascoupled thereto a means for providing a carrier gas stream and a meansfor providing an electron beam at said operating pressure and furthercontains an evaporant source, wherein said means for providing anelectron beam comprises a differentially pumped electron beam gun havingan electron beam generating portion to which is pumped by a high vacuumturbomolecular pump to provide a vacuum of greater than 8×10⁻⁵ Torrduring operation, and a differential pumping source in connection with aportion of the electron beam gun immediately adjacent to the electronbeam generating portion to remove gas from the carrier gas stream whichenters an end of the electron beam gun contained inside said depositionchamber; impinging said evaporant source with said electron beam togenerate said evaporant; entraining said evaporant in said carrier gasstream; and coating said substrate with said carrier gas stream whichcontains said entrained evaporant.